The present invention relates to a tire for vehicle wheels of the type provided with a tread band comprising: a plurality of raised blocks each peripherally confined between at least two longitudinal grooves substantially extending circumferentially of the tire, and at least two transverse grooves joining said longitudinal grooves; a first series of sipes and a second sipes formed in said blocks, each according to an inclined orientation relative to a direction normal to an outer rolling surface of the tread band, to divide each block into a plurality of flexible portions disposed circumferentially in side by side relationship, each having a grip edge in the form of an acute angle on said outer rolling surface.
In more detail, the invention is directed to a tire of the so-called xe2x80x9cwinterxe2x80x9d type and to a method of making such a tire, the tread band of which has a tread pattern particularly suitable for running on snow-covered and/or frozen roadways.
Tires of the winter type are required to have, in addition to excellent behavioural features as regards running on a snow-covered roadway, also a good qualitative level in connection with other operation features such as rolling smoothness, running behaviour on a wet surface and kilometric yield.
Other features that, although of less importance than the preceding ones, also help in giving a better or worse qualitative evaluation of a winter tire are running behaviour on a dry surface and quietness on running.
The above mentioned behavioural and operational features are determined by making appropriate circumferential and transverse grooves into the tread band, which grooves must be suitably sized and oriented, so as to give rise to formation of blocks normally aligned in rows disposed consecutively in side by side relationship and extending circumferentially of the tire.
In addition, the presence of an appropriate lamelliform pattern in the blocks, i.e. a thick series of narrow cuts disposed consecutively in side by side relationship in a circumferential direction and oriented substantially transversely of the rolling direction is significant indispensable as regards achievement of particular behavioural running features on a snow-covered roadway.
The task of these narrow cuts, more simply called xe2x80x9csipesxe2x80x9d is substantially that of picking up and efficiently retaining the snow, since friction created by snow against snow is, as known, greater than friction of rubber on snow.
In U.S. Pat. No. 4,298,046, herein mentioned as an example of the state of the most pertinent prior art, it is described a winter tire the tread band of which has four circumferential rows of blocks, confined by substantially straight circumferential grooves and transverse grooves, straight as well, having walls substantially normal to an outer rolling surface, on which contact between the tread band and roadway surface occurs.
For the purpose of achieving improvements in terms of traction power and grip performance on braking it is provided that blocks belonging to the axially outer rows, located close to the tire shoulders, should have sipes oriented according to an inclination of about 30xc2x0 relative to the rolling surface and extending on the rolling surface itself according to a substantially straight extension parallel to the tire rotation axis. Blocks belonging to the circumferential rows placed in the middle region of the tire have sipes similar to those formed in the blocks belonging to the shoulder rows, but inclined in the opposite direction relative to the latter.
In each block, sipes define a plurality of flexible portions that, due to the inclination of the sipes themselves, exhibit a sharp-corner edge capable of exerting an efficient grip on the roadway surface. By assigning a given direction of rotation to the tire, the acute-angle edges of the flexible portions belonging to the blocks of the shoulder rows are made efficient during the acceleration step, while the acute-angle edges of the flexible portions of the blocks belonging to the centre rows exert their efficiency during the braking step.
In Patent EP 498287 it is described a tire in which the transverse grooves delimiting the tread band blocks have an inclined orientation relative to a direction normal to the rolling surface, in order to improve evenness in wear of the blocks themselves. The transverse grooves belonging to the blocks of the circumferential shoulder rows have an opposite orientation relative to that of the blocks belonging to the centre rows. In addition a different direction of rotation is assigned to the tire, depending on whether it is associated with a driving wheel or a driven wheel of the motor-vehicle.
In patent EP 384182 and in the Italian Patent Application MI94A000309 tires for heavy duty vehicles are described which are provided with circumferential rows of blocks delimited by transverse cuts of reduced width having an inclined orientation relative to a direction normal to the rolling surface of the tread band.
In accordance with the present invention it has been found advantageous for the blocks in the tread band to be provided with sipes having an inclined orientation relative to the rolling surface and a curvilinear transverse extension, or in any case such arranged as to define a concavity turned towards the acute-angle grip edge exhibited by one of the two flexible portions delimited by the sipe itself. Thus the grip effect on traction and/or braking is improved, above all on a snow-covered and/or frozen roadway, in that the consequent conformation of each flexible portion makes this portion substantially behave like a small shovel cutting and picking up the snow and accumulating and holding it at the inside of the sipe.
In more detail, it is an object of the present invention to provide a tire for vehicle wheels, characterized in that each sipe extends transversely of the block according to a concave extension towards the grip edge of the corresponding flexible portion.
Preferably, sipes belonging to the first series and the second series are distributed in respective circumferential rows disposed axially in side by side relationship and advantageously arranged in an alternate sequence.
More particularly, each circumferential row of sipes belonging to the first and second series respectively is preferably associated with a respective circumferential row of blocks.
Advantageously, each sipe may be provided to extend transversely of the block substantially over the whole width thereof.
Each sipe may possibly have at least one end portion of reduced depth close to the longitudinal groove delimiting the respective block.
In a preferential embodiment, each sipe has a substantially curvilinear extension symmetric with the longitudinal centre line of the respective block.
In addition, each of said flexible portions may be provided to have opposite ends inclined to the extension direction of the corresponding longitudinal grooves, to define side toothings projecting into the longitudinal grooves in said block.
In accordance with a further aspect of the invention application of which can also take place independently of the conformation of the transverse extension of the sipes, it has been found advantageous for each of said transverse grooves to be confined between a first side substantially normal to the outer rolling surface of the tread band, and a second side diverging from the first side towards the rolling surface itself.
Preferably said second side, relative to the rolling surface, has an orientation similar to or concordant with that of the sipes of the corresponding block.
It may be also provided that each block should have, close to said rolling surface, at least one transverse edge projecting towards the corresponding transverse groove to define an undercut with the second side of the transverse groove itself.
Advantageously, each of said transverse grooves extends in a direction substantially parallel to the extension of the sipes, and the transverse grooves belonging to blocks disposed mutually in side by side relationship in an axial direction are disposed consecutively so as to define substantially continuous transverse grooves extending according to an undulated extension.
It may be conveniently provided that each of said longitudinal grooves should be confined between a first side substantially normal to the rolling surface and a second side diverging from the first side in the direction of the rolling surface itself.
Each block may also have, close to the rolling surface, at least one longitudinal edge projecting towards the corresponding longitudinal groove, to define an undercut with the second side of the longitudinal groove itself.
Preferably, in each of said longitudinal grooves the mutual positioning of the first and second sides is inverted relative to the mutual positioning of the first and second sides of the circumferentially contiguous longitudinal grooves.
Conveniently, the longitudinal grooves are consecutively disposed so as to define substantially continuous circumferential grooves.
Preferably, each of said longitudinal grooves has an inclined extension relative to a direction circumferential to the tire, so that each of said circumferential grooves extends in the form of a broken line.
In a preferential embodiment of the tire in accordance with the invention, at least two circumferential rows of shoulder blocks are provided which extend close to the opposite side edges of the tread band, as well as one or more circumferential rows of centre blocks disposed symmetrically relative to an equatorial plane of the tire, the blocks disposed on either side of the equatorial plane respectively having correspondingly opposite orientations.
In accordance with a different aspect, the invention relates to a method of increasing snow trapping in tread patterns for tires of vehicle wheels, comprising the steps of: making rows of blocks, each confined between longitudinal and transverse grooves relative to a circumferential extension direction of the tire; dividing each block into a plurality of flexible portions by a plurality of sipes extending inwardly of the tire starting from a radially outer rolling surface; shaping said sipes into a concave line on said rolling surface; and inclining each sipe preferably at an angle included between 20xc2x0 and 35xc2x0 relative to a straight line normal to the rolling surface.